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The Tunguska Event

The Tunguska Event, or Tunguska explosion, was a powerful explosion that occurred over the Southern swamp, a small morass not far from the Podkamennaya (Lower Stony) Tunguska River in what is now Krasnoyarsk Krai in Russia, at 0 hours 13 minutes 35 seconds Greenwich Mean Time (around 7:14 a.m. local time) on June 30, 1908 (June 17 in the Julian calendar, in use locally at the time).

Although the cause of the explosion is the subject of debate, it is commonly believed to have been caused by the air burst of a large meteoroid or comet fragment at an altitude of 5–10 kilometres (3.1–6.2 mi) above the Earth’s surface. Different studies have yielded varying estimates of the object’s size, with general agreement that it was a few tens of metres across.

The whole number of scholarly publications on the problem of the Tunguska explosion between 1908 and 2009 may be estimated as about 1000 (mainly in Russian). Many scientists have participated in Tunguska studies, the most well-known of them being Leonid Kulik, Yevgeny Krinov, Kirill Florensky, Nikolay Vasiliev, and Wilhelm Fast.

Although the meteor or comet burst in the air rather than hitting the surface, this event is still referred to as an impact. Estimates of the energy of the blast range from 5 megatons of TNT (21 PJ) to as high as 30 megatons of TNT (130 PJ), with 10–15 megatons of TNT (42–63 PJ) the most likely— roughly equal to the United States’ Castle Bravo thermonuclear bomb tested on March 1, 1954, about 1,000 times as powerful as the atomic bomb dropped on Hiroshima, Japan and about one-third the power of the Tsar Bomba, the largest nuclear weapon ever detonated. The explosion knocked over an estimated 80 million trees over 2,150 square kilometres (830 sq mi). It is estimated that the shock wave from the blast would have measured 5.0 on the Richter scale. An explosion of this magnitude is capable of destroying a large metropolitan area. This possibility has helped to spark discussion of asteroid deflection strategies.

The Tunguska event is the largest impact event over land in Earth’s recent history. Impacts of similar size over remote ocean areas would have gone unnoticed before the advent of global satellite monitoring in the 1960s and 1970s.

At around 7:17 a.m. local time, Tungus natives and Russian settlers in the hills northwest of Lake Baikal observed a column of bluish light, nearly as bright as the Sun, moving across the sky. About 10 minutes later, there was a flash and a sound similar to artillery fire. Eyewitnesses closer to the explosion reported the sound source moving east to north. The sounds were accompanied by a shock wave that knocked people off their feet and broke windows hundreds of miles away. The majority of eyewitnesses reported only the sounds and the tremors, and not the sighting of the explosion. Eyewitness accounts differ as to the sequence of events and their overall duration.

The explosion registered on seismic stations across Eurasia. In some places the shock wave would have been equivalent to an earthquake of 5.0 on the Richter scale. It also produced fluctuations in atmospheric pressure strong enough to be detected in Great Britain. Over the next few days, night skies in Asia and Europe were aglow such that those in London could read a newspaper by their light; it has been theorized that this was due to light passing through high-altitude ice particles formed at extremely cold temperatures, a phenomenon that occurs when the Space Shuttle re-enters the Earth’s atmosphere. In the United States, the Smithsonian Astrophysical Observatory and the Mount Wilson Observatory observed a decrease in atmospheric transparency that lasted for several months, from suspended dust.

Selected eyewitness reports

Testimony of S. Semenov (pronounced SemiONov), as recorded by Leonid Kulik’s expedition in 1930.
“At breakfast time I was sitting by the house at Vanavara Trading Post (65 kilometres/40 miles south of the explosion), facing north. […] I suddenly saw that directly to the north, over Onkoul’s Tunguska Road, the sky split in two and fire appeared high and wide over the forest (as Semenov showed, about 50 degrees up — expedition note). The split in the sky grew larger, and the entire northern side was covered with fire. At that moment I became so hot that I couldn’t bear it, as if my shirt was on fire; from the northern side, where the fire was, came strong heat. I wanted to tear off my shirt and throw it down, but then the sky shut closed, and a strong thump sounded, and I was thrown a few yards. I lost my senses for a moment, but then my wife ran out and led me to the house. After that such noise came, as if rocks were falling or cannons were firing, the earth shook, and when I was on the ground, I pressed my head down, fearing rocks would smash it. When the sky opened up, hot wind raced between the houses, like from cannons, which left traces in the ground like pathways, and it damaged some crops. Later we saw that many windows were shattered, and in the barn a part of the iron lock snapped.”

Testimony of Chuchan of Shanyagir tribe, as recorded by I.M. Suslov in 1926.
“We had a hut by the river with my brother Chekaren. We were sleeping. Suddenly we both woke up at the same time. Somebody shoved us. We heard whistling and felt strong wind. Chekaren said, ‘Can you hear all those birds flying overhead?’ We were both in the hut, couldn’t see what was going on outside. Suddenly, I got shoved again, this time so hard I fell into the fire. I got scared. Chekaren got scared too. We started crying out for father, mother, brother, but no one answered. There was noise beyond the hut, we could hear trees falling down. Chekaren and I got out of our sleeping bags and wanted to run out, but then the thunder struck. This was the first thunder. The Earth began to move and rock, wind hit our hut and knocked it over. My body was pushed down by sticks, but my head was in the clear. Then I saw a wonder: trees were falling, the branches were on fire, it became mighty bright, how can I say this, as if there was a second sun, my eyes were hurting, I even closed them. It was like what the Russians call lightning. And immediately there was a loud thunderclap. This was the second thunder. The morning was sunny, there were no clouds, our Sun was shining brightly as usual, and suddenly there came a second one!
“Chekaren and I had some difficulty getting out from under the remains of our hut. Then we saw that above, but in a different place, there was another flash, and loud thunder came. This was the third thunder strike. Wind came again, knocked us off our feet, struck against the fallen trees.
“We looked at the fallen trees, watched the tree tops get snapped off, watched the fires. Suddenly Chekaren yelled ‘Look up’ and pointed with his hand. I looked there and saw another flash, and it made another thunder. But the noise was less than before. This was the fourth strike, like normal thunder.
“Now I remember well there was also one more thunder strike, but it was small, and somewhere far away, where the Sun goes to sleep.”

Sibir newspaper, July 2, 1908
“On the 17th of June, around 9am in the morning, we observed an unusual natural occurrence. In the north Karelinski village (200 verst, or about 130 miles, north of Kirensk) the peasants saw to the north west, rather high above the horizon, some strangely bright (impossible to look at) bluish-white heavenly body, which for 10 minutes moved downwards. The body appeared as a “pipe”, i.e. a cylinder. The sky was cloudless, only a small dark cloud was observed in the general direction of the bright body. It was hot and dry. As the body neared the ground (forest), the bright body seemed to smudge, and then turned into a giant billow of black smoke, and a loud knocking (not thunder) was heard, as if large stones were falling, or artillery was fired. All buildings shook. At the same time the cloud began emitting flames of uncertain shapes. All villagers were stricken with panic and took to the streets, women cried, thinking it was the end of the world.

“The author of these lines was meantime in the forest about 6 verst (about four miles) north of Kirensk, and heard to the north east some kind of artillery barrage, that repeated in intervals of 15 minutes at least 10 times. In Kirensk in a few buildings in the walls facing north east window glass shook.”

Siberian Life newspaper, July 27, 1908
“When the meteorite fell, strong tremors in the ground were observed, and near the Lovat village of the Kansk uezd two strong explosions were heard, as if from large-caliber artillery.”

Krasnoyaretz newspaper, July 13, 1908
“Kezhemskoe village. On the 17th an unusual atmospheric event was observed. At 7:43 the noise akin to a strong wind was heard. Immediately afterwards a horrific thump sounded, followed by an earthquake which literally shook the buildings, as if they were hit by a large log or a heavy rock. The first thump was followed by a second, and then a third. Then the interval between the first and the third thumps were accompanied by an unusual underground rattle, similar to a railway upon which dozens of trains are travelling at the same time. Afterwards for 5 to 6 minutes an exact likeness of artillery fire was heard: 50 to 60 salvoes in short, equal intervals, which got progressively weaker. After 1.5 – 2 minutes after one of the “barrages” six more thumps were heard, like cannon firing, but individual, loud and accompanied by tremors.

“The sky, at the first sight, appeared to be clear. There was no wind and no clouds. However upon closer inspection to the north, i.e. where most of the thumps were heard, a kind of an ashen cloud was seen near the horizon which kept getting smaller and more transparent and possibly by around 2-3 p.m. completely disappeared.”

There was little scientific curiosity about the impact at the time, possibly due to the isolation of the Tunguska region. If there were any early expeditions to the site, the records were likely to have been lost during the subsequent chaotic years — World War I, the Russian Revolution of 1917 and the Russian Civil War.

The first recorded expedition arrived at the scene more than a decade after the event. In 1921, the Russian mineralogist Leonid Kulik, visiting the Podkamennaya Tunguska River basin as part of a survey for the Soviet Academy of Sciences, deduced from local accounts that the explosion had been caused by a giant meteorite impact. He persuaded the Soviet government to fund an expedition to the Tunguska region, based on the prospect of meteoric iron that could be salvaged to aid Soviet industry. Kulik’s party eventually undertook an expedition in 1927.

Upon arrival, Kulik made arrangements with the local Evenki hunters to guide his party to the impact site. Reaching the explosion site was an extremely arduous task. Upon reaching an area just south of the site, the superstitious Evenki hunters would go no further, fearing what they called the Valleymen. Kulik had to return to the nearby village, and his party was delayed for several days while they sought new guides.

The spectacle that confronted Kulik as he stood on a ridge overlooking the devastated area was overwhelming. To the explorers’ surprise, no crater was to be found. There was instead around ground zero a vast zone (8 kilometers across) of trees scorched and devoid of branches, but standing upright. Those farther away had been partly scorched and knocked down in a direction away from the centre. Much later, in the 1960s, it was established that the zone of leveled forest occupied an area of some 2150 square kilometers, its shape resembling a gigantic spread-eagled butterfly with a “wingspan” of 70 kilometers and a “body length” of 55 kilometers.

Upon closer examination, Kulik located holes which he erroneously concluded were meteorite holes; however, he did not have the means at this time to excavate the holes.

During the next ten years there were three more expeditions to the area. Kulik found several dozens of little “pothole” bogs, each some 10 to 50 meters in diameter, that he thought might be meteoric craters. After a laborious exercise in draining one of these bogs (the so-called “Suslov’s crater”, 32 meters in diameter), he found there was an old stump on the bottom, ruling out the possibility that it was a meteoric crater. In 1938, Kulik arranged for an aerial photographic survey of the area covering the central part of the leveled forest (some 250 square kilometers). The negatives of these aerial photographs (1500 negatives, each 18 x 18 cm) were burned in 1975 by order of Evgeny Krinov, then Chairman of the Committee on Meteorites of the USSR Academy of Sciences.

Despite the large amount of devastation, there was no crater to be seen.

Expeditions sent to the area in the 1950s and 1960s found microscopic silicate and magnetite spheres in siftings of the soil. Similar spheres were predicted to exist in the felled trees, although they could not be detected by contemporary means. Later expeditions did identify such spheres in the resin of the trees. Chemical analysis showed that the spheres contained high proportions of nickel relative to iron, which is also found in meteorites, leading to the conclusion they were of extraterrestrial origin. The concentration of the spheres in different regions of the soil was also found to be consistent with the expected distribution of debris from a meteorite airburst. Later studies of the spheres found unusual ratios of numerous other metals relative to the surrounding environment, which was taken as further evidence of their extraterrestrial origin.

Chemical analysis of peat bogs from the area also revealed numerous anomalies considered consistent with an impact event. The isotopic signatures of stable carbon, hydrogen, and nitrogen isotopes at the layer of the bogs corresponding to 1908 were found to be inconsistent with the isotopic ratios measured in the adjacent layers, and this abnormality was not found in bogs located outside the area. The region of the bogs showing these anomalous signatures also contains an unusually high proportion of iridium, similar to the iridium layer found in the K–T boundary. These unusual proportions are believed to result from debris from the impacting body that deposited in the bogs. The nitrogen is believed to have been deposited as acid rain, a suspected fallout from the explosion.

Meteoroid airburstIn scientific circles, the leading explanation for the explosion is the airburst of a meteoroid 6–10 kilometres (4–6 miles) above Earth’s surface.

Meteoroids enter Earth’s atmosphere from outer space every day, usually travelling at a speed of more than 10 kilometres per second (6 miles/sec or 21,600 mph). Most are small but occasionally a larger one enters. The heat generated by compression of air in front of the body (ram pressure) as it travels through the atmosphere is immense and most meteoroids burn up or explode before they reach the ground. Since the second half of the 20th century, close monitoring of Earth’s atmosphere has led to the discovery that such meteoroid airbursts occur rather frequently. A stony meteoroid of about 10 metres (30 ft) in diameter can produce an explosion of around 20 kilotons, similar to that of the Fat Man bomb dropped on Nagasaki, and data released by the U.S. Air Force’s Defense Support Program indicate that such explosions occur high in the upper atmosphere more than once a year. Tunguska-like megaton-range events are much rarer. Eugene Shoemaker estimated that such events occur about once every 300 years.

Blast patternsThe explosion’s effect on the trees near ground zero was replicated during atmospheric nuclear tests in the 1950s and 1960s. These effects are caused by the shock wave produced by large explosions. The trees directly below the explosion are stripped as the blast wave moves vertically downward, while trees further away are knocked over because the blast wave is travelling closer to the horizontal when it reaches them.

Soviet experiments performed in the mid-1960s, with model forests (made of matches on wire stakes) and small explosive charges slid downward on wires, produced butterfly-shaped blast patterns strikingly similar to the pattern found at the Tunguska site. The experiments suggested that the object had approached at an angle of roughly 30 degrees from the ground and 115 degrees from north and had exploded in mid-air.

Asteroid or comet?
The composition of the Tunguska body remains a matter of controversy. In 1930, the British astronomer F.J.W. Whipple suggested that the Tunguska body was a small comet. A cometary meteorite, being composed primarily of ice and dust, could have been completely vaporized by the impact with the Earth’s atmosphere, leaving no obvious traces. The comet hypothesis was further supported by the glowing skies (or “skyglows” or “bright nights”) observed across Europe for several evenings after the impact, possibly explained by dust and ice that had been dispersed from the comet’s tail across the upper atmosphere. The cometary hypothesis gained a general acceptance amongst Soviet Tunguska investigators by the 1960s.

In 1978, astronomer ?ubor Kresák suggested that the body was a fragment of the short-period Comet Encke, which is responsible for the Beta Taurid meteor shower; the Tunguska event coincided with a peak in that shower, and the approximate trajectory of the Tunguska impactor is consistent with what would be expected from such a fragment. It is now known that bodies of this kind explode at frequent intervals tens to hundreds of kilometres above the ground. Military satellites have been observing these explosions for decades.

In 1983, astronomer Zden?k Sekanina published a paper criticizing the comet hypothesis. He pointed out that a body composed of cometary material, travelling through the atmosphere along such a shallow trajectory, ought to have disintegrated, whereas the Tunguska body apparently remained intact into the lower atmosphere. Sekanina argued that the evidence pointed to a dense, rocky object, probably of asteroidal origin. This hypothesis was further boosted in 2001, when Farinella, Foschini, et al. released a study suggesting that the object had arrived from the direction of the asteroid belt.

Proponents of the comet hypothesis have suggested that the object was an extinct comet with a stony mantle that allowed it to penetrate the atmosphere.

The chief difficulty in the asteroid hypothesis is that a stony object should have produced a large crater where it struck the ground, but no such crater has been found. It has been hypothesized that the passage of the asteroid through the atmosphere caused pressures and temperatures to build up to a point where the asteroid abruptly disintegrated in a huge explosion. The destruction would have to have been so complete that no remnants of substantial size survived, and the material scattered into the upper atmosphere during the explosion would have caused the skyglows. Models published in 1993 suggested that the stony body would have been about 60 metres across, with physical properties somewhere between an ordinary chondrite and a carbonaceous chondrite.

Christopher Chyba and others have proposed a process whereby a stony meteorite could have exhibited the behavior of the Tunguska impactor. Their models show that when the forces opposing a body’s descent become greater than the cohesive force holding it together, it blows apart, releasing nearly all its energy at once. The result is no crater, and damage distributed over a fairly wide radius, all of the damage being blast and thermal.

3D numerical modelling the Tunguska impact, done by Utyuzhnikov and Rudenko in 2008, supports the comet hypothesis. According to their results, the comet matter dispersed in the atmosphere, while the destruction of the forest was caused by the shock wave.

During the 1990s, Italian researchers extracted resin from the core of the trees in the area of impact, to examine trapped particles that were present during the 1908 event. They found high levels of material commonly found in rocky asteroids and rarely found in comets.

In research published in the journal Geophysical Research Letters, scientists contend that the impact was caused by a comet due to the sightings of noctilucent clouds following the impact, a phenomenon caused by massive amounts of water vapor in the upper atmosphere. They compared the noctilucent cloud phenomenon to the exhaust plume from the NASA space shuttle Endeavour.

Lake ChekoIn June 2007 it was announced that scientists from the University of Bologna had identified a lake in the Tunguska region as a possible impact crater from the event. They do not dispute that the Tunguska body exploded in mid-air, but believe that a one meter fragment survived the explosion and impacted the ground. Lake Cheko is a small bowl-shaped lake approximately 8 kilometres north-north-west of the hypocenter. The hypothesis has been disputed by other impact crater specialists. A 1961 investigation had dismissed a modern origin of Lake Cheko, saying that the presence of metres thick silt deposits at the lake’s bed suggests an age of at least 5,000 years; but more recent research suggests that only a meter or so of the sediment layer on the lake bed is “normal lacustrine sedimentation”, a depth indicating a much younger lake, about 100 years. Acoustic-echo soundings of the lake floor provide support for the hypothesis that the lake was formed by the Tunguska event. The soundings revealed a conical shape for the lake bed, which is consistent with an impact crater. Magnetic readings indicate a possible meter-sized chunk of rock below the lake’s deepest point, which may be a fragment of the colliding body. Finally, the lake’s long axis points to the hypocenter of the Tunguska explosion, about 7.0 km away. Work is still being done at Lake Cheko to determine its origins.

Scientific understanding of the behaviour of meteorites in the Earth’s atmosphere was much sparser during the early decades of the 20th century. Due to this lack of knowledge, as well as a lack of scientific data about Tunguska due to Soviet secrecy during the Cold War, a great many other hypotheses for the Tunguska event have sprung up, none of which are accepted by the scientific community.

Comet 2005NB56One study “suggests that a chunk of Comet 2005NB56 caused the 5-10 megaton fireball, bouncing off the atmosphere and back into orbit around the sun.” The scientists involved in the study claim that the object that caused the event will pass close to Earth again in 2045.

End of the world?

“ Perhaps the earliest widely-held theory for the Tunguska explosion was that the world was about to end. As the minutes passed, this theory was dropped in favour of other, less final theories, until today one is hard-pressed to find anyone who truly believes the world ended on the morning of 30 June 1908…”

According to G. K. Kulesh, head of the Kirensk Meteorological Station

“ The peasants of the village [of Karelino] were so stunned by the crashes that they sent a deputation to town to the local archpriest to ask if the end of the world was beginning, [and] how they were preparing for it in Kirensk. ”

Natural H-bombIn 1989, Serge J.D. D’Alessio and Archie A. Harms suggested that some of the deuterium in a comet entering the Earth’s atmosphere may have undergone a nuclear fusion reaction, leaving a distinctive signature in the form of carbon-14. They concluded that any release of nuclear energy would have been almost negligible. Independently, in 1990, César Sirvent proposed that a deuterium comet, i.e., a comet with an anomalous high concentration of deuterium in its composition, could have exploded as a natural hydrogen bomb, generating most of the energy released. The sequence would be first a mechanical or kinetic explosion, triggering a thermonuclear reaction. These proposals are inconsistent with our knowledge of the composition of comets and of the temperature and pressure conditions necessary for initiating a nuclear fusion reaction. Studies have found the concentration of radioactive isotopes in the blast region to be inconsistent with those expected following a nuclear explosion, fusion or otherwise.

Black HoleIn 1973, Albert A. Jackson and Michael P. Ryan, physicists at the University of Texas, proposed that the Tunguska event was caused by a small (around 1017 kg to 1019 kg) black hole passing through the Earth. This hypothesis is flawed, as there was no so-called exit event — a second explosion occurring as the black hole, having tunnelled through the Earth, shot out the other side on its way back into space. Based on the direction of impact, the exit event would have occurred in the North Atlantic, closer than the impact event to the seismic recording stations that collected much of the evidence of the event. The hypothesis also fails to account for evidence that cosmic material was deposited by the impacting body, including dust trails in the atmosphere and the distribution of high-nickel magnetic spheres around the impact area.

AntimatterIn 1941, Lincoln LaPaz, and later in 1965, Clyde Cowan, Chandra R. Atluri, and Willard F. Libby suggested that the Tunguska event was caused by the annihilation of a chunk of antimatter falling from space. As with the other hypotheses described in this section, this does not account for the mineral debris left in the area of the explosion.

The Wardenclyffe TowerOliver Nichelson suggested that the Tunguska explosion may have been the result of an experiment by Nikola Tesla using the Wardenclyffe Tower, performed during one of Admiral Robert Peary’s North Pole expeditions.

Alien spaceship crashA number of UFO conspiracy theories have claimed that the Tunguska event was the result of the activities of extraterrestrial beings, including an exploding alien spaceship or even an alien weapon going off to “save the Earth from an imminent threat”. These claims appear to originate from a science fiction story “The Explosion” written by the Soviet engineer Alexander Kazantsev in 1946, in which a nuclear-powered Martian spaceship, trying to land on the Earth, meets with a disaster and blows up in mid-air. Kazantsev never visited Hiroshima, but his idea of the above-ground explosion of the Tunguska space body was inspired by the news about the nuclear explosion over that Japanese city, as well as by his talks with some leading Soviet nuclear physicists.

Many events in Kazantsev’s tale, which was intended as pure fantasy, were subsequently confused with the actual occurrences at Tunguska. The nuclear-powered UFO hypothesis was adopted by the TV critics Thomas Atkins and John Baxter in their book The Fire Came By (1976). The television series The Secret KGB UFO Files (Phenomenon: The Lost Archives) in 1998, broadcast on Turner Network Television, referred to the Tunguska event as “the Russian Roswell” and claimed that crashed UFO debris had been recovered from the site. In 2004, a group from the Tunguska Spatial Phenomenon Foundation claimed to have found the wreckage of an alien spacecraft at the site. In 2009, Dr Yuri Labvin, the president of the Tunguska Spatial Phenomenon Foundation repeated these claims, based upon findings of quartz slabs with strange markings on them found at the site, which, he claims, represent the remnants of an alien spaceship’s control panel.

Geophysical hypothesisAstrophysicist Wolfgang Kundt has suggested the Tunguska event was caused by the sudden release and subsequent explosion of 10 million tons of natural gas from within the Earth’s crust. The similar Verneshot hypothesis has also been suggested as a possible cause of the Tunguska event.